Comprehensive optical monitoring of photopolymer curing for additive manufacturing of diffractive elements

Diffractive optical elements (DOE) offer a wide range of possibilities, from beam shaping to augmented reality and neural networks. Additive manufacturing using photopolymerization curing shows great potential for manufacturing customizable DOEs at low cost. To design and fabricate these, it is essential to monitor both the dynamic evolution of the curing process as well as its spatial distribution during and after curing. To this end, we propose an all-optical monitoring platform comprising a "focused line refractive index microscopy" (FLRIM) technique based on total internal reflection at a prism interface and a large field-of-view interferometric imager, more specifically a "lateral-shearing interferometric microscopy" (LIM) technique. The FLRIM enables dynamic in-situ spatio-temporal quantitative measurements of refractive index (RI) during the curing process, while the LIM technique provides quantitative information of 2D structures by measuring the transmitted phase with high sensitivity via multi-angle illumination. An ultraviolet "digital micromirror device" (DMD) projector is used to photopolymerize pixelated 2D structures. By using the proposed monitoring platform, we investigate the resulting photopolymer structures in-situ during several local curing steps, and, additionally, before and after a global post-curing. Besides showing the capabilities of our technology, we additionally demonstrate that there is potential to fabricate DOEs on a single photopolymer by exploiting tunable local RI changes.